System for sensing a sample

ABSTRACT

A profiler or scanning probe microscope may be scanned across a sample surface with a distance between them controlled to allow the sensing tip to contact the surface intermittently in order to find and measure features of interest. The distance is controlled so that when the sensing tip is raised or lowered to touch the sample surface, there is no lateral relative motion between the tip and the sample. This prevents tip damage. Prior knowledge of the height distribution of the sample surface may be provided or measured and used for positioning the sensing tip initially or in controlling the separation to avoid lateral contact between the tip and the sample. The process may also be performed in two parts: a fast find mode to find the features and a subsequent measurement mode to measure the features. A quick step mode may also be performed by choosing steps of lateral relative motion to be smaller than 100 nanometers to reduce probability of tip damage. In this mode, after each vertical step to increase the separation between the tip and the sample, it is detected as to whether the tip and the sample are in contact. If they are still in contact after the vertical step, one or more vertical steps are taken to increase the separation, and no vertical step to reduce the separation is taken and no lateral relative motion is caused until it is determined that the tip and the sample are no longer in contact.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 08/30,641, filed Oct. 11, 1996, entitled “Dual Stage Instrument forScanning a Specimen” which, in turn, is a continuation-in-part ofapplication Ser. No. 08/598,848, filed Feb. 9, 1996.

BACKGROUND OF THE INVENTION

[0002] Stylus profilers are used for obtaining surface profiles ofsamples. The stylus of the profiler follows the surface under a smallcontact force, and the resulting motions of the stylus are measured witha sensor assembly. The sensor assembly includes a stylus, a mechanicallinkage (usually a stylus arm) connecting the stylus to a flexure pivot,and a transducer. When the stylus is scanned across the surface of thesample, the force exerted by the sample surface on the stylus causes arotation of the stylus arm about the flexure pivot. The verticaldisplacement of the stylus is converted by the transducer into anelectrical signal which indicates the profile of the sample surface.

[0003] Advanced profilers also include a force control mechanism, suchas an electromagnetic actuator, for maintaining a constant contact forcebetween the stylus and the sample surface as the stylus is scannedacross the surface. To maintain a constant contact force between thestylus and the sample surface, the spring action of the flexure pivot iscalibrated and the force control magnetic actuator is controlled tocounteract the change in the force applied by the flexure spring on thestylus caused by rotation of the stylus arm. Thus, a constant force isexerted by the stylus against the sample surface, as the stylus isscanned across the surface. As an example of a profiler which has beenused in the semiconductor and disk drive industries, please see U.S.Pat. No. 5,705,741 and U.S. Pat. No. 5,309,755; both patents areincorporated herein in their entirety by reference.

[0004] As the semiconductor industry progresses to smaller dimensionswith each new generation of products, there is an increasing need forscanning instruments that can measure sub-micrometer scale surfacefeatures. While the depths or vertical dimensions (dimensions normal tothe plane of the wafer surface) of the features such as trenches, or viaholes, in semiconductor wafers, commonly exceed one micrometer, thelateral dimensions (dimensions in the plane of the wafer surface) havebeen continually reduced. At the current state of the art, the lateraldimensions of features such as trenches are less than 0.5 micrometer.With the continual reduction of the lateral dimensions of features suchas trenches and via holes in the surface of semiconductor wafers, theratio of depth to the lateral dimension of such features, also known asthe aspect-ratio, is continually increased.

[0005] In order to measure such high aspect-ratio features, a verysharp, thin but long (high aspect-ratio) stylus must be used. However, asharp, thin but long stylus is fragile and may easily break, especiallywhen subjected to lateral forces (forces in directions in, or parallelto, the plane of the sample surface). Thus, when a high aspect-ratiostylus contacts a steep feature, such as the side wall of a trench orvia hole, the contact force has a relatively large lateral component anda relatively small vertical component. Stylus profilers, such as theprofilers described in the two patents referenced above, are designedsuch that motion of the stylus is constrained to one degree of freedom,namely, rotation about the flexure pivot. This degree of freedom issubstantially normal to the sample surface. The stylus arm is relativelystiffin all other degrees of freedom. Consequently, the lateral forcesgenerated when the high aspect-ratio stylus encounters a steep wall caneasily break the stylus and damage the sample being measured.

[0006] The stylus arm in a profiler has a single degree of freedom,which comprises rotations about a pivot. The stylus or sensing tiptravels along a path normal to a radial line passing through the centerof rotation at the pivot and the tip. Since the sensing or stylus tipmust be located “below” or at a lower elevation than the pivot to ensurethat the tip and not the body of the sensor assembly contacts thesample, the motion of the stylus or sensing tip is not truly normal tothe plane of the sample surface, but is in the shape of an arc. Whilethe main direction of travel of the tip is downwards, it neverthelessalso travels in the lateral direction in the plane of the samplesurface. This lateral motion is also known as parasitic motion of thesensing tip. The parasitic motion of the sensing tip may hamper or evenpreclude the sensor assembly from measuring relatively deep and narrowfeatures.

[0007] It is therefore desirable to provide an improved surfacemeasurement system which overcomes the above drawbacks.

SUMMARY OF THE INVENTION

[0008] The above-described difficulties can be overcome by allowing thesensing tip of the profiler to contact the sample surface withoutsubstantially rotating the stylus arm about the pivot. Instead, adistance between the sample and the sensing tip of the profiler isreduced until the tip touches the sample, without moving the tip and thesample laterally relative to each other. By avoiding lateral relativemotion between the tip and the sample before the tip touches thesurface, the above-described problems are avoided. When such a scanningprocess is used, thin and long (high aspect-ratio) styli can be used topenetrate high aspect-ratio features for measurement. Data related tothe height of the sample may then be measured with the tip stationaryand in contact with the sample. After the measurement, the tip and thesample are separated and moved laterally relative to each other tomeasure the sample surface at a different location.

[0009] With minor modifications, the above-described scanning processmay also be applied to other scanning instruments, such as the scanningprobe microscope, which includes atomic force microscopes and scanningtunneling microscopes.

[0010] As described above, the feature on the sample surface may befound and measured by repeatedly causing the sensing tip (of theprofiler or scanning probe microscope, for example) and a sample torepeatedly contact at different locations of the sample surface. In thisprocess, the sensing tip and the sample are brought togethersubstantially without lateral relative motion between them until theycontact, separated again substantially without lateral relative motionbetween them, and moved laterally relative to each other until the tipis at a location above a different portion of the sample. This processis repeated at different locations of the sample. If the separationbetween the tip and the sample during such lateral motion is less thanthe change in height of the sample surface, the lateral motion willcause the sensing tip to contact the sample surface laterally, therebycausing damage to the sensing tip. To reduce the probability of suchdamage, the separation may be increased to a large value before lateralrelative motion is initiated. If no knowledge of the height variation ordistribution of the sample surface is available, such value should belarge enough that it exceeds any probable height variations of thesample surface one may encounter. The resulting process can be quitetime consuming, especially if the sample surface is to be measured atmany different locations. This difficulty can be avoided by separatingthe tip and the surface by just enough to avoid such lateral contact.

[0011] A number of techniques may be employed to assure that the sensingtip and the sample are separated by an adequate distance so that thesensing tip will not contact the sample surface during the subsequentlateral relative motion. In the preferred embodiments, if certain heightinformation is provided concerning the sample surface or a portionthereof (such as within a target area), then the sensing tip may bepositioned at or close to the portion of the sample having the highestelevation. If the height information of the sample surface or a portionthereof is not readily available, such information can be acquiredquickly by actually measuring the height of the surface at severalsampling locations. Yet another technique that can be employed is toactually measure data related to the height of the sample when the tipand the sample come into contact as the tip is scanned across the sampleand use such measured data to predict an elevation of the next locationof the sample to be measured, so that the separation between the tip andthe sample can be set to be higher than such predicted elevation. Theseare, of course, only some examples of the techniques that can be used toimplement the above general concept.

[0012] In some applications, it may be desirable to first find thefeature quickly, and then take an appropriate amount of time to actuallymeasure the feature. In this instance, the distance between the sensingtip and the sample is controlled so that the distance between the tipand the sample is periodically increased and then decreased as the tipscans across the sample surface until the tip either touches the surfaceor until either the tip or the surface has traveled, or the two togetherhave traveled in aggregate, by a preset distance without causing the tipand the surface to contact. In other words, the sensing tip does notcompletely penetrate the feature when scanning across the surface.

[0013] For some applications, to save time, even without any priorknowledge concerning the topology of the sample surface, the samplesurface can be quickly scanned and measured without incurring undue riskin breaking the sensing tip. This involves determining whether the tipand the surface remain in contact after the distance between them isincreased to a predetermined value, before lateral relative motionbetween them is initiated or continued. If the tip and. the surfaceremain in contact after they are being separated from each other by apredetermined distance, the distance between them is further increaseduntil they are no longer in contact before moving the tip and thesurface laterally with respect to each other.

[0014] A sensor assembly having a sensing probe may be used in any oneof the above-described processes for sensing the sample. The sensorassembly includes a base portion and a moveable sensing tip connected tothe base portion. When the tip contacts the sample, the tip may moverelative to the base portion of the sensor assembly. A moving stage isused to cause vertical relative motion between the sensor assembly andthe sample. Thus, when the moving stage causes a distance between thesensor assembly and a sample to be reduced until the sensing tipcontacts the sample, the actual change in distance between the sensingtip and the sample is given by a combination of the relative motionbetween the sensor assembly and the sample, and of the relative motionbetween the sensing tip and the base portion. By taking into accountboth motions, a more accurate measure of data related to the height ofthe sample can be obtained. In different embodiments, the sensorassembly may be that of a profiler, an atomic force microscope or othertypes of scanning probe microscopes.

[0015] One way to increase measurement speed while avoiding significantlateral forces between the sensing tip and the sample is to separate theprocess into two parts: an initial fast find mode to find the feature ofinterest, and after finding the feature, a second measurement mode tomeasure the feature.

[0016] As one possible embodiment of the invention to implement theabove two part process, the sensing tip is scanned across the samplesurface with the tip in contact with the surface until the feature isfound. Scanning the sensing tip across the surface with the tip incontact with the surface speeds up the scanning process. After thefeature is found, the sensing tip is scanned across the feature with thetip in intermittent contact with the sample surface to measure thefeature. In this manner, the time required to find and measure thefeature is reduced without undue risk of large lateral forces betweenthe sensing tip and the sample surface. In the preferred embodiment, twodifferent styli with known tip offsets are used in this process. Thefirst stylus is used to scan while in contact with the surface to findthe feature. The second stylus is then used to measure the feature.

[0017] In yet another aspect of the invention, a sensor assembly havinga sensing probe with a sensing tip is employed. When the sensing tip ofthe probe is used for sensing a sample, the vertical distance moved bythe sensor assembly (or by both the assembly and the surface inaggregate) until the tip contacts the surface within a feature ofinterest may be taken as the depth of the feature. To determine that thetip has contacted the surface, the sensor assembly is driven towards thesurface until the distance moved by the tip relative to the assemblyexceeds a threshold, at which point the vertical relative motion betweenthe tip and the surface is stopped and the vertical distance moved bythe assembly (or the sum of the distances moved by both the assembly andsurface) is noted to indicate the depth of the feature. To yield a moreaccurate measure of such depth, the motion of the probe tip relative tothe assembly is taken into account in calculating such depth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1A is a schematic view of a prior art stylus sensor assembly,similar to that of U.S. Pat. No. 5,309,755.

[0019]FIG. 1B is a schematic view of how a high aspect-ratio sensing tipmay be destroyed by large lateral forces generated when the sensing tipcontacts a side wall in a conventional profiling scheme.

[0020]FIG. 1C is a schematic view of a sensing tip of the stylus sensorassembly and the high aspect-ratio feature illustrating the arcuatemotion of the stylus, or its parasitic motion, which prevents the tipfrom reaching the bottom of high aspect-ratio features in a conventionalprofiling scheme.

[0021]FIG. 1D is a schematic view illustrating how the probe of thestylus sensor assembly may be lowered without rotation in order to reachthe bottom of high aspect-ratio features.

[0022]FIG. 2A is a schematic view of a profiler which includes thestylus sensor assembly of FIG. 1A controlled by a digital signalprocessor and mounted to both a Z stage and an XY stage to illustratethe preferred embodiment of this invention.

[0023]FIG. 2B is a schematic view of an atomic force microscope (AFM)with an XYZ stage for moving the sample to illustrate this invention.

[0024]FIG. 3 is a schematic view of the scanning path of the sensing tipin the profiler of FIG. 2A to illustrate one embodiment of theinvention.

[0025]FIG. 4A is a schematic view of the scanning path of the sensingtip in the profiler of FIG. 2A or the AFM in FIG. 2B to illustrate afast find mode of this invention.

[0026]FIG. 4B is a schematic view of the scanning path of the sensingtip in the profiler of FIG. 2A or in the AFM in FIG. 2B to illustrateanother embodiment of the fast find mode of this invention.

[0027]FIG. 5 is a schematic view of the scanning path of the sensing tipin the profiler or in the AFM of FIGS. 2A and 2B along an inclinedsurface of the sample with a feature to illustrate another aspect of theinvention in the fast find mode.

[0028]FIG. 6 is a schematic view of the scan path of the sensing tip inthe profiler or in the AFM in FIGS. 2A and 2B to illustrate the fastprofiling mode, where the tip is positioned using knowledge of thesurface height distribution of the surface to be scanned.

[0029]FIG. 7A is a schematic view of the scan path of the sensing tip inthe profiler or in the AFM of FIGS. 2A and 2B in a quick step mode ofthis invention where no surface height distribution information isutilized.

[0030]FIG. 7B is a schematic view of a scan path of the sensing tip inthe profiler or in the AFM of FIGS. 2A and 2B in a quick step mode ofthis invention, illustrating how the sensing tip senses an inclined sidewall.

[0031] FIGS. 8A-8C are cross-sectional views of a surface inintermittent search paths employing the sensing tip of FIG. 2A or 2B toillustrate another embodiment of the invention. This embodiment is takenfrom parent application Ser. No. 08/730,641.

[0032]FIG. 9A is a schematic view of a stylus arm and two sensing tipsto illustrate another embodiment of the invention.

[0033]FIG. 9B is a simplified perspective view of a portion of a stylussensor assembly with two probes, each probe having a sensing tip toillustrate another embodiment of the invention.

[0034] For simplicity in description, identical components areidentified with the same numerals in this application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035]FIG. 1A is a simplified schematic view of a stylus sensor assemblysimilar to the one of U.S. Pat. No. 5,309,755 useful for illustratingthis invention. As shown in FIG. 1A, stylus sensor assembly 10 includesa base portion which includes the support body 12 and a transducersupport 14 rigidly attached together. The support body 12 has a pivot 16therein equipped with a spring so that the pivot 16 is a flexure pivot.Connected rotatably to the flexure pivot 16 is one end of a stylus arm18 having a sensing tip or stylus 20 at or near the other end of thestylus arm. The stylus arm 18 is connected to a vane 22 on the otherside of the pivot 16. Therefore, when the stylus 20 contacts a surfaceand is caused to move up or down, causing the stylus arm 18 to alsorotate, vane 22 will rotate as well, thereby causing a change in thecapacitance between the two capacitor plates 24 which are supported bythe transducer support 14. The change in capacitance is fed to a digitalsignal processor (shown in FIG. 2A) which computes the distance rotatedby stylus 20 from the change in capacitance.

[0036] The stylus arm 18 is biased downwards so that stylus 20 applies aforce against the surface that is being profiled. This biasing isaccomplished by means of a force coil 26 through which a current ispassed. The amount of current passed through the force coil is such thatstylus 20 applies a predetermined force against a surface duringprofiling. When the tip is rotated up or down by the sample, the flexurespring is stretched or compressed, where such stretching or compressionapplies a variable force in addition to the force applied by the currentin the force coil. As a further improvement as set forth in U.S. Pat.No. 5,705,741, the effect of the flexure spring when the tip is rotatedabout pivot 16 is calibrated so that the amount of current applied tothe force coil 26 is altered as a function of the displacement of stylus20, to thereby substantially cancel out the effect of the flexure springon the force applied by the tip 20 to the sample. In this manner, theforce applied by stylus 20 to the sample surface remains constantdespite rotation and displacement of the stylus.

[0037]FIG. 1B illustrates a possible scenario where a high aspect-ratiostylus 20 of the stylus sensor assembly 10 is used to profile a highaspect-ratio feature, such as a deep trench or via hole. As shown inFIG. 1B, stylus or sensing tip 20 scans the surface 40 of a sample, bycausing lateral relative motion between the stylus 20 and surface 40,shown along the X axis in FIG. 1B. When the sensing tip 20 enters a highaspect-ratio feature, such as a trench or via hole 42, large lateralforces (forces in directions substantially along or parallel to thesample surface 40, such as along the X axis) will be generated, whichmay cause a portion of the stylus or sensing tip 20 to break off when itcontacts the side wall of the feature.

[0038] As noted above, stylus 20 is rotated about pivot 16 by the samplesurface 40 when it is scanned across the sample surface. Therefore,stylus 20 travels along a path 44 which is curved or arcuate in shape.For high aspect-ratio features, such arcuate motion of the stylus orsensing tip 20 may make it impossible for the sensing tip to reach thebottom of the trench or hole 42.

[0039] This invention is based on the recognition that, in order toavoid the problems described above in reference to FIGS. 1B, 1C, insteadof rotating stylus or sensing tip 20 about pivot 16 in order to measurethe bottom of high aspect-ratio features, the stylus 20 is simplylowered vertically into the feature 42 without rotating the stylus 20 orstylus arm 18 about pivot 16 as illustrated in FIG. 1D. As shown in FIG.1D, if the stylus 20 is simply lowered into the feature 42 withoutmoving the tip and the sample laterally relative to each other andwithout substantially rotating the arm about the pivot, then it ispossible for stylus or sensing tip 20 to reach and measure the bottom offeature 42.

[0040] In order to lower the stylus 20 into the feature, stylus sensorassembly 10 is mounted onto a Z stage 62 and an XY stage 64 asillustrated in FIG. 2A. FIG. 2A is a schematic view of a profiler whichincludes the stylus sensor assembly 10 of FIG. 1A controlled by adigital signal processor 52 and mounted to both a Z stage and an XYstage to illustrate the preferred embodiment of this invention. As shownin FIG. 2A, the transducer support 14 of the base portion of stylussensor assembly 10 is attached to the Z stage 62 which, in turn, isattached to the XY stage 64. Therefore, the stylus 20 may be loweredinto the feature 42 of FIG. 1D by means of Z stage 62, by lowering theentire stylus sensor assembly 10. In this manner, the above-describedproblems associated with the operation in FIGS. 1B, 1C can be avoided. Zstage 62 raises or lowers the stylus sensor assembly 10 without movingthe stylus 20 laterally relative to the sample surface 40. The XY stage64 causes lateral relative motion, for example, in the X direction,between the stylus 20 and the sample surface 40; the X direction issubstantially parallel to the sample surface 40.

[0041]FIG. 3 is a schematic view of a scan path of the stylus tip 20 ofFIG. 2A for finding and measuring a feature in the surface to illustrateone embodiment of the invention. Thus, stylus sensor assembly 10 ispositioned so that stylus 20 is at position 72 above a location 74 ofthe sample surface 40. Stylus 20 is then lowered by means of Z stage 62by lowering the entire stylus sensor assembly until it is determinedthat stylus 20 touches or is in contact with surface 40 at location 74.Stylus 20 is then raised, again by means of Z stage 62 by raising theentire stylus sensor assembly 10 until the stylus 20 is again at thestarting point 72. The XY stage 64 then causes lateral relative motionbetween the sample surface and the stylus 20, by moving the stylussensor assembly 10 and the Z stage 62 along the X direction by apredetermined step size dx to point 76 which is above another location78 of the sample surface 40, where location 78 is spaced apart from butadjacent to location 74. The Z stage 62 is again used to lower stylus 20by lowering the stylus sensor assembly 10 until it is determined thatstylus 20 is in contact with the surface 40 at location 78. Stylus 20(and the stylus sensor assembly) is then raised by means of Z stage 62back to point 76 and lateral relative motion between the sample surfaceand stylus 20 is again caused by XY stage 64. The lateral motion stepsize dx may be in a range of about 1 nm to 50 mm.

[0042] It is noted from the above description that when the distanceseparating the stylus or sensing tip 20 and the sample surface 40 isincreased or decreased, there is substantially no lateral relativemotion between the sample surface and the stylus. Points 72, 76 . . . towhich the stylus 20 is raised after contact with the sample surface areat a distance of Z2 above sample surface 40. In the embodiment of FIG.3, after each time the stylus contacts the surface 40, the stylus israised by the same amount Z2 above the location of the sample surface itwas in contact with prior to the raising. Therefore, as will be shownbelow, if this distance Z2 is greater than the depth of any feature thatthe stylus may encounter during the scanning motion of the stylus acrosssample surface 40, then the stylus would not come into lateral contactwith any side walls of the sample surface 40 to cause the type ofproblems described above in reference to FIGS. 1B, 1C.

[0043] Thus, the above process of lowering, raising and lateral movementin reference to FIG. 3 is repeated until the stylus is at point 82 abovelocation 84 within a feature 42 a of the sample surface. The Z stage 62lowers the stylus sensor assembly and the stylus until the styluscontacts the sample surface at location 84 within the feature 42 a. Thenwhen the Z stage 62 raises the stylus by distance Z2, since Z2 isgreater than the depth Z of any feature on the sample surface 40,including the depth of feature 42 a, after the Z stage 62 raises thestylus to point 86, point 86 will be at an elevation higher than samplesurface 40. Therefore, when the XY stage 64 causes lateral relativemotion between the sample surface 40 and stylus 20 along the X axis,stylus 20 will not come into contact with any side walls of feature 42a. Thus, the scanning process continues until the stylus reaches point88 where it is above location 90 on surface 40 outside feature 42 a. TheZ stage 62 lowers stylus 20 by a short distance when it comes intocontact with surface 40 at location 90. Then again, Z stage 62 raisesthe stylus to point 92 by a distance of Z2 above location 90 before theXY stage 64 again causes lateral relative motion between the samplesurface 40 and stylus 20, to repeat and continue the above-describedintermittent contact scanning across surface 40. Obviously, in all ofthe embodiments of this application, instead of moving the stylus sensorassembly of the profiler (or of AFM, SPM) to cause relative motionbetween the sensing tip and the sample surface, such relative motion canbe caused by moving the sample instead or a combination of motions ofthe sample and of the sensing tip; all such variations are within thescope of the invention.

[0044] In reference to FIG. 2A, when stylus 20 comes into contact withsurface 40, surface 40 will cause stylus arm 18 and stylus 20 to berotated about pivot 16 while Z stage 62 continues to lower stylus sensorassembly 10 towards the surface 40, while system 60 is determiningwhether the tip 20 has contacted the sample surface. In order to becertain that stylus 20 has indeed come into contact with surface 40, itis preferable for a threshold distance to be set, and it is determinedthat stylus 20 has come into contact with surface 40 only after thedistance rotated by stylus 20 about pivot 16 has exceeded the thresholddistance. Such threshold distance may be a parameter that can be setthrough the digital signal processor 52. Thus, when the change incapacitance across capacitance plates 24 caused by rotation of stylusarm 18 and vane 22 indicates that a distance rotated by stylus 20 isequal to or has exceeded the threshold distance, the digital signalprocessor 52 will send a signal along one of the lines 54 to Z stage 62to cause the Z stage to stop lowering the stylus sensor assembly 10.

[0045] Since the stylus sensor assembly of the profiler continues to belowered towards the sample after the tip 20 touches the sample and thelowering motion is stopped only after the tip has been rotated by athreshold distance, the stylus sensor assembly has been lowered by adistance greater than the distance traveled by the sensing tip, by thedistance rotated by the tip. In order to measure the height of surface40, the Z stage 62 records the distance that the stylus sensor assembly10 has been lowered until it is determined that stylus 20 has come intocontact with surface 40. If such distance is taken as the distancebetween the starting point of stylus 20 and the end point in thelowering process, such as the distance between point 72 and 74, suchdistance may actually be greater than the distance traveled by stylus20, by the distance that is rotated by stylus 20, after the tip contactsthe surface but before the Z stage 62 stops lowering stylus sensorassembly 10. The DSP 52 sends a signal along one of the lines 54 to thecontroller (not shown) of the Z stage 62 to indicate the actual distancerotated by stylus 20 before motion of the Z stage is stopped and thecontroller of the Z stage will then subtract such distance from thedistance that the Z stage 62 has moved stylus sensor assembly 10, toobtain a more accurate measure of the actual distance traveled by stylus20. This will give a more accurate measure of data related to the heightof sample surface 40 and of the profile of any features of interest inthe sample surface.

[0046] The methods of this invention described herein for finding andmeasuring surface features may be carried out by means of SPMs insteadof profilers. In such event, instead of raising or lowering a stylussensor assembly, one would raise or lower a SPM sensor assembly. Wherean AFM is used instead of a profiler stylus sensor assembly as thesensing probe, a threshold distance can also be defined to determinewhen the probe tip has come into contact with the sample surface. FIG.2B is a schematic view of an AFM with an XYZ stage 91 for moving thesample to illustrate this invention. As shown in FIG. 2B, the amount ofbending of the cantilever arm 92 is monitored in a conventional manner(such as by sensing the strain in the arm 92, or by the amount ofbending of the arm), and controller 94 causes the probe or the arm to belifted or lowered to maintain a constant strain or amount of bending inthe arm. If the tip 92 a of arm 92 has come into contact with the samplesurface, this will cause a strain and bending of the arm. In otherwords, when the sensing probe carrying the sensing tip or stylusapproaches the sample surface, if the tip comes into contact with thesurface, the continued motion of the probe towards the surface willcause the arm to bend and strain to develop in the arm, where the straindeveloped and the amount of bending will correspond to the distancemoved by the probe and sensor assembly after the tip is in contact withthe surface. Thus a threshold distance may be set which corresponds to avalue for the strain or bending of the arm. By sensing whether thestrain or bending of the arm has exceeded such value, it is possible todetermine whether the tip 92 a has come into contact with samplesurface. The same process can be performed for other SPMs such as thescanning tunneling microscope, by setting a threshold value for thecurrent between the tip and the sample, for example.

[0047] Measurements of surface profiles and depths of features using theSPM can be improved in accuracy in the same manner as that describedabove. Where the sensing tip of a AFM is used in the depth measurement,for example as shown in FIG. 2B, the probe 92 and tip 92 a are caused toapproach the surface until the tip 92 a touches the surface, and theprobe is driven further towards the surface until it is determined thatthe strain in the probe or amount of bending of the probe has reached acertain threshold. The distance moved by the tip relative to the SPMprobe positioner 96 (which forms a base portion of the AFM sensorassembly, not completely shown in FIG. 2B) when the sensor assembly isdriven towards the surface should be taken into account (e.g. bysubtracting such distance from the depth measurement) when calculatingthe depth of a feature.

[0048] While the process or mode of operation illustrated in FIG. 3 maybe advantageous for some applications, it may be time consuming andcumbersome for other applications, since the stylus must be raised to adistance greater than the expected height variation or heightdistribution of sample surface 40. This is true especially where thesample surface 40 is inclined or tilted. In such circumstance, or inothers where one is unsure of the amplitude of height variations of thesample surface, in order to avoid lateral contact between the stylus andany side walls, the stylus must be raised to relatively large heightsabove the sample surface 40. This may be time consuming and cumbersome.

[0049] In order to avoid having to raise the stylus by distances muchlarger than the actual height variations of the sample surface, it willbe useful to have some prior knowledge of the height distribution of thesample surface (e.g. within a target area) before scanning starts. Forexample, if the portion or point of the highest elevation of the samplesurface is known, the stylus 20 or 20′ may be positioned at a pointdirectly above or close to such highest point or portion before scanningstarts. Then such starting point and the distance by which the tip issubsequently raised above the prior point of contact with the samplesurface can be much reduced. In one embodiment, such distance can be ina range of about 100 to 500 nanometers. Then the above-describedprocedure in reference to FIG. 3 may be carried out without the risk ofthe stylus coming into lateral contact with a side wall of the samplesurface, where Z2 can be reduced to the sum of the expected featuredepth and a shorter distance such as one in a range of about 100 to 500nanometers.

[0050] When prior knowledge of the sample surface 40 is not availablebefore scanning starts, it may be a simple and fast procedure to obtainsuch height distribution information by carrying out the process asillustrated in FIG. 3, with Z2 at a large value, but only at a fewsampling locations of surface 40, such as 3 to 25 locations. Since theheights of only a few locations of the sample surface 40 are measured,this process will not take an inordinate amount of time even when alarge value of Z2 is used. Typically, the user is able to position thestylus sensor assembly over the general area of a feature of interest,such as at a point that is above a surface location within about 1 or 2microns from the feature of interest. Therefore, a target area ofseveral microns by several microns (e.g. 2 by 2 microns) may be defined,and the several sampling locations chosen within the target area. Theabove-described process in reference to FIG. 3 may be carried out onlyat such locations in such small target area to find out the heightdistribution over such area. After the height distribution of the targetarea is known, then the stylus may be positioned at a point which isabove a portion of the sample surface which is at or close to the pointof highest elevation in the distribution. It should be noted that, evenif the stylus is not placed immediately above the point of the highestelevation of the target area, as long as the distance by which thestylus is raised after contacting such point causes the stylus to behigher in elevation than any portion of the sample surface within thetarget area, the stylus will not come into lateral contact with anyportion of the sample surface to damage the stylus in the subsequentlateral relative motion between the tip and the surface. This allows theuser a higher tolerance in positioning the tip.

[0051] The total time required for finding and measuring features ofinterest on a sample surface can be further reduced by separating theprocess into two parts: a first fast find mode in which the stylus doesnot necessarily completely penetrate the features of interest so as tofirst find the features, and a second measuring mode for measuring thefeatures found. This permits the stylus to be raised or lowered by shortdistances during the fast find mode in order to find the feature, and asubsequent measurement mode in which the stylus penetrates the featuresby touching the sample surface at one or more locations within a featureof interest for measuring data related to the height of the feature andof the surrounding sample surface. This is illustrated in reference toFIGS. 4A, 4B.

[0052]FIG. 4A is a schematic view of a scan path illustrating a fastfind mode for finding a feature. In the embodiment of FIG. 4A, it isassumed that information concerning height variations of sample surface40 is available or has been measured, so that such information can beused to position the sensing tip of a stylus sensor assembly or SPMabove the sample surface 40. Such positioning is particularly simplewhere the sample surface is characterized by two discreet levels orheights as illustrated in FIG. 4A. Thus, outside of the feature 42 a,the sample surface is at a particular height, and within the feature,the sample surface has two side walls 42 a(s) and a bottom portion 42a(b) at a depth Z below the top surface. In such event, as long as thestarting position of the sensing tip is above the top surface by acertain distance, and the sensing tip is raised to the same elevation asthe starting position above the sample surface each time the sensing tipis raised and separated from the sample surface, the sensing tip willnot come into lateral contact with the sample surface such as side wall42 a(s).

[0053] To speed up the process of scanning in the fast find mode, the Zstage 62 lowers the sensing probe of the stylus sensor assembly or ofthe SPM until either the sensing tip of the probe touches the surface 40of the sample or has been lowered by a preset distance. In one example,the sensing tip is placed a short distance (e.g. approximately 100-500nanometers) above the top (upper) surface of the sample at the startingpoint. In a process similar to that described above in reference to FIG.3, the sensing tip is lowered until it is determined that the sensingtip has come into contact with the sample surface without anysubstantial lateral relative motion between the sensing tip and thesample surface, raised again after such contact also without anysubstantial lateral relative motion therebetween to a point, movedlaterally across the surface of the sample and the process repeated overa target area of the sample surface, similar to the process of FIG. 3.

[0054] The sample surface 40 has two discrete levels: a top level ofsurface portion 40 t and the level of the bottom surface 42 a(b) withinthe feature 42 a. In this example, the sensing tip is raised by a shortdistance in a range of about 100-500 nanometers. In one embodiment, thetip is raised to about 300 nanometers above the top surface after eachcontact with the top sample surface 40 t, except when the sensing tip isdirectly above bottom surface 42 a(b) of the feature 42 a, at position104. When the sensing tip is lowered into the hole or trench 42 a, thelowering of the sensing probe is stopped after the probe has beenlowered by a preset distance even though it is determined that thesensing tip has not come into contact with the sample surface. This isdetermined by the fact that the tip has not been caused to rotate by atleast the threshold distance as described above, for example. In thesame example, such preset distance may be 450 nanometers. In theembodiment of FIG. 4A, where the sample surface is at two discreetlevels, this means that the sensing tip will be lowered to a position106 approximately 150 nanometers below the top discreet level of thesample surface 40 t at which point the lowering motion of the sensingprobe would be stopped. The sensing tip is then again raised by the Zstage 62. In the embodiment of FIG. 4A, it is raised by the samedistance each time, whether or not the sensing tip has contacted thesample surface, or in other words, by 300 nanometers. This means thatwhen the sensing tip is scanning immediately above feature 42 a, thesensing tip will be raised to only 150 nanometers above the top level ofthe sample surface 40 t, to position 108. This process is repeated untilthe sensing tip is at position 110 at which point the XY stage 64 movesthe sensing probe along the X axis to a position 112. Since points 110and 112 are still approximately 150 nanometers above the top surface 40t of the sample (FIG. 4A not drawn to scale), the sensing tip will notcome into contact with the sample during this lateral relative motion.The sensing tip may then again be lowered by the Z stage which lowersthe entire sensing probe until the tip touches the surface at position114. The tip is then raised to position 116 and the process is thenrepeated across the remainder of the sample surface within the targetarea.

[0055] The location of the feature can then be determined by recordingthe XY positions of the points at which the sensing tip was lowered bythe preset distance but did not come into contact with the samplesurface. After such process, the system is operated in a measurementmode in which the probe is lowered into feature 42 a until it isdetermined that the sensing tip has come into contact with the bottom 42a(b) of the feature of 42 a, in order to measure data related to theheight or depth of the bottom surface of the feature. Since features ofinterest are typically small, the measurement mode is normally not verytime consuming.

[0056]FIG. 4B is a schematic view of a scan path and of a sample surfacesimilar to those illustrated in FIG. 4A, but where the sensing tip andthe sensing probe are raised by a greater distance when they areimmediately above the feature than when they are not. In the embodimentof FIG. 4B, for example, the sensing tip is raised to substantially thesame elevation throughout the scan across the surface in the targetarea. Thus, as in the example of FIG. 4A, when the sensing tip and probeare above the sample surface but not above the feature 42 a, the sensingtip is raised to approximately 300 nanometers about the top surface ofthe sample. Where the probe is immediately above feature 42 a, the probeis raised so that the sensing tip is retrieved to the same elevation asthe starting point 102. If the sensing tip is allowed to be lowered intothe feature beneath the top surface of the sample by 150 nanometers (orlowering the tip by a total of 450 nm) to point 122 at which point the Zstage stops lowering the probe and sensing tip, then the Z stage wouldraise the probe and sensing tip by such preset distance which is 450nanometers so that at point 124, the sensing tip is at the sameelevation as the starting position 102. Aside from such difference, thescanning mode in FIG. 4B is substantially the same as that illustratedabove in reference to FIG. 4A.

[0057] The scanning process described above in reference to FIGS. 4A and4B is quite effective where the sample surface is level and the Z stage62 raises and lowers the sensing probe and tip in directionssubstantially normal to the sample surface. In other words, the Zdirection of motion of the Z stage 62 is substantially perpendicular tothe sample surface 40. Where this assumption is not true, as would bethe case where the sample surface is tilted with respect to the Zdirection of motion of the Z stage, or where the sample surface has aportion that is inclined and has a slope, the above-described schemesmay still cause the sensing tip to come into lateral contact with thesample surface, which may result in tip damage. Applicants havedeveloped a technique to avoid such contact as described above inreference to FIG. 5.

[0058] As shown in FIG. 5, the sample surface is inclined with a certainslope. Thus, if the scanning process described above in reference toFIGS. 4A, 4B are employed, when the sensing tip is in or above thefeature 42 b and scanning along the dotted line 132, the sensing tip maycome into contact with the side wall 42 b(s) of the feature 42 b,thereby causing tip damage. The above scenario can be avoided byactually measuring the slope of the top surface 40 t′ during the fastfind mode and use such data to predict the elevation of the nextlocation of the sample surface to be sampled. Thus, from data related tothe height of the top surface 40 t′ at the various locations of thesurface taken before the sensing tip reaches the feature 42 b, a slopeof the top surface 40 t may be derived by means of the digital signalprocessor 52. Such slope may then be used to predict the elevation ofanother portion of the top sample surface 40 t′ yet to be sampled by thesensing probe. Therefore, even though the sensing tip does not come intocontact with the sample surface when it is immediately above feature 42b, such slope information is used and extrapolated so that the scanningpath follows the general slope of the top surface 40 t and so that whenthe scanning probe reaches the vicinity of side wall 42 b(s), thesensing tip will be at an elevation above the side wall and not comeinto contact with it, as shown by the solid line scan path in FIG. 5.Where the top surface 40 t′ of the sample is relatively flat, its slopecan be calculated readily. Where the top surface 40 t′ of the sample isnot flat, the elevation of the next location of the sample surface to besampled may be predicted by a curve fitting process which is known tothose skilled in the art and will not be elaborated herein for thatreason.

[0059] In some applications, it may be desirable to use informationconcerning height distribution of the surface that is provided orobtained as described above, and then simply find and measure featuresof interest within a target area, without separating the process into afinding process and a measuring process. This is illustrated in FIG. 6.Thus, as in the case of FIG. 4A, height information of the surface 40 isemployed to position the sensing tip at a location 102 above the samplesurface, where one is certain that the starting position of the sensingtip is at an elevation higher than any part of the target area of thesample to be measured. One can then be certain that the sensing tip willnot come into lateral contact with the sample surface to cause tipdamage. In the scanning process across the sample surface, theseparation between the sensing tip and the sample surface is controlledso that the sensing tip comes into contact intermittently with thesample surface, where the sensing tip is lowered to contact the samplesurface, and raised to the same elevation as the starting position 102after each contact with the sample surface, without causing relativelateral motion between the sensing tip and the sample. Lateral relativemotion is caused only when the sensing tip has been raised to the sameelevation as that of the starting point 102. In other words, lateralmotion is caused only when the sensing tip is at an elevation higherthan all points in the target area of the sample surface so that thesensing tip will not come into lateral contact with the sample surfacewhile lateral relative motion is caused between the sensing tip and thesample.

[0060] Thus, from the above several embodiments, it will be seen that auseful method for sensing a feature on the surface of a sample employinga sensing tip has been described. The sensing tip is positioned aboveone location of the surface. A distance between the surface and thesensing tip is reduced without substantially moving the tip and thesurface laterally relative to each other. In the embodiment of FIG. 6,for example, the distance is reduced until the tip touches the surface.In the embodiments of FIGS. 4A, 4B and 5, the distance is reduced untilthe tip touches the surface or until the tip or the surface hastraveled, or the tip and the surface together have traveled inaggregate, by a preset distance without the tip contacting the surface.A distance or separation between the tip and the surface is thenincreased without substantially moving the tip and the surface laterallyrelative to each other until such distance is substantially equal to apredetermined value. The predetermined value is such that after suchseparation has been increased to the predetermined value, the tip ishigher in elevation than another location of the surface adjacent to andspaced apart from the one location. (This predetermined value may be onethat is calculated and predicted from a measurement of the samplesurface, such as that illustrated above in reference to FIG. 5 where theslope of the top surface 40 t is measured. Alternatively, suchpredetermined value may be arrived at by using knowledge of the heightdistribution of the sample surface, such as by measuring at a fewsampling locations the height distribution of the sample surface withina target area as described above.) Lateral relative motion is thencaused between the sensing tip only after the tip is raised by adistance equal to such predetermined value from the surface of thesample and the tip is positioned so that it is above another location.The steps immediately described above of reducing the distance,increasing the distance and causing lateral relative motion are thenrepeated at a plurality of locations of the surface to find or measurethe feature. In the embodiments of FIGS. 4A, 4B, during the fast findmode, the height of the sample surface is not measured. In theembodiment of FIGS. 5 and 6, on the other hand, data related to theheight of the sample surface is measured. During such measurements, itis preferable for the sensing tip to be stationary and in contact withthe sample. In the preferred embodiment, the predetermined value forjudging how far to raise the tip may be not more than about 1 micron.

[0061] For many applications, it is desirable to quickly scan thesurface to find and measure any features of interest without undue riskof damage to the tip and of obtaining false data. This is known as thequick step mode as illustrated in FIGS. 7A, 7B. In this mode, thesensing tip of a stylus sensor assembly (of a profiler or SPM) is placedat a small distance above one location of the sample, such as onebetween 100 and 500 nanometers. In one embodiment, the tip is raised toaround 300 nanometers above the sample. The sensing tip is then scannedacross the surface of the sample with the distance separating the sampleand the tip controlled so that the sensing tip contacts the sampleintermittently as it is scanned across the sample surface 40. After eachcontact with the sample, the sensing tip is raised to a small distanceabove the surface of the sample preferably without any substantiallateral relative motion between them, before lateral relative motion iscaused between the sample and the tip. Such small distances may be thesame during each raise and may be substantially the same as the initialseparation, namely, about 300 nanometers. This mode of scanning runs therisk of the sensing tip contacting a side wall such as 42 a(s) atlocation 152 as shown in FIG. 7A but if the step size of the lateralrelative motion is chosen to be small, the sensing tip or stylus of thestylus sensor assembly or SPM may be able to stand the stress generatedby the side wall impact at location 152. Thus, the mode of operation inthe quick step mode of FIG. 7A is similar to that shown in FIG. 3,except that the vertical distances raised by the Z stage 62 may not beadequate to avoid lateral contact with the side wall. The quick stepmode of FIG. 7A, however, differs from that in FIG. 3 in that it isfurther determined whether the sensing tip and the surface are incontact after a vertical step has been taken to increase the distancebetween the sensing tip and the sample.

[0062] After a vertical step has been completed, the desired lateralrelative motion is made. The tip is then made to approach the surface.If the tip rises relative to the sensor assembly as soon as the approachbegins, no gap between the tip and the surface had resulted from theinitial vertical step. This can be determined since the movement of thetip is monitored. Alternatively, the amount of movement of the tiprelative to the sensor assembly can be monitored when the tip approachesthe surface to determine whether the distance moved by the tip relativeto the assembly has exceeded the set threshold as described above. Ineither case, another vertical step to increase the vertical separationbetween the sensing tip and the sample is carried out, at which point itis again determined whether the tip and the surface are in contact. Inreference to FIG. 7A, for example, when the sensing tip contacts theside wall 42 a(s) at location 152, the sensing tip will attempt to movelaterally but will stay in contact with the side wall at location 152.The sensing system will then determine whether the tip and the surfaceare still in contact. Since the tip and the surface at location 152 arestill in contact at such moment, any downward motion of the sensorassembly will cause the stylus to move relative to the assembly by thesame amount and the downward motion is terminated. The sensor assemblyis instead raised by another vertical step to location 154. At suchlocation, the measurement system determines that the sensing tip and thesample are still in contact. The sensing tip is therefore again raisedvertically to position 156 in contact with the side wall. This processis then repeated until the sensing tip reaches the position 162 afterthe vertical step, at which point the measuring system determines thatthe sensing tip and surface 40 are no longer in contact. At this point,the sensing tip is again lowered to come into contact with samplesurface 40 at location 164 and raised again to position 162 before it ismoved laterally as before to perform intermittent contact andmeasurement of the top surface 40 t of the sample. In order to reducethe probability of tip damage, the steps of lateral relative motion maybe over a lateral distance less than about 100 nanometers. FIG. 7Billustrates essentially the same process as in FIG. 7A, but as appliedto a sample surface with an inclined surface portion.

[0063] In the conventional mode of operation of the stylus sensorassembly of FIG. 1A, a desirable tracking force is applied by applyingan appropriate current through the force coil 26 so that stylus 20applies a desired force against the sample surface that is beingscanned. In the different modes of operation described above for thisinvention, however, the stylus or sensing tip of the stylus sensorassembly would start at a position not in contact with the samplesurface. A preset value of a desired tracking force is set at the DSP52. To improve the stability of scanning, an initial current is appliedto the force coil 26 in FIG. 2A to compress the spring in flexure pivot16 and to hold the stylus 20 at a steady position while the stylussensor assembly 10 is being lowered and raised by the Z stage 62 ortransported laterally by means of stage 64. When the stylus 20 touchesthe sample surface, a force will develop between the tip and the sample.Such force may increase as the tip is rotated above pivot 16 due to theaction of the spring. If the force exerted by the spring caused by itsstretching or compression is precalibrated as described in U.S. Pat. No.5,705,741, then the amount of force between the tip and the sample canbe found by the amount of rotation of the stylus 20 about pivot 16. DSP52 compares such force to a preset value stored at the DSP. When theforce between the tip and the sample reaches the preset value, DSP 52applies a control signal to a power supply (not shown) to change theamount of current applied to the force coil 26, so as to maintain theforce between the tip and the sample substantially at the preset valuewhen the tip is rotated further by the sample until the distance thatthe tip is raised is substantially equal to the threshold value asexplained above. The threshold value may be set to a distance of notmore than about 500 nanometers. In this manner, the threshold value fordetermining whether the probe tip has come into contact with the samplesurface can be set by setting a value for a corresponding force betweenthe tip and the sample at the DSP 52. Similarly, a threshold value fordetermining whether the probe tip has come into contact with the samplesurface can be set in the case of AFM or other SPM by setting a valuefor a corresponding force between the tip and the sample at thecontroller 94.

[0064] FIGS. 8A-8C together with their accompanying description beloware taken from the parent application.

[0065] In order to measure the profile or geometry of a surface, inreference to FIG. 8A, system 20 lifts the probe tip by a predetermineddistance h from the surface, record the lateral distance δx traveled bythe tip before it is lowered again to touch the surface and record thedistance by which the probe tip has been lowered before it touches thesurface again. Preferably, the tip is again lifted from such point ofcontact by the distance h, moved laterally by distance δx, lowered againto touch the surface, and the distance that the tip is lowered againrecorded. This process is then repeated until the scan across the targetarea is completed. A record of such distance δx and the distances thatthe tip is repeatedly lowered before it touches the surface in theintermittent contact mode throughout the scan will give an indication ofthe geometry or profile of the surface.

[0066] In the embodiment of FIG. 8A, the probe tip is lifted after it islowered to touch the surface 200, without dragging the probe tip alongthe surface. In other words, the probe tip is caused to gently tapsurface 200 before it is lifted and the probe tip is not moved laterallyacross the surface while it is in contact with the surface. In someapplications, it may be desirable to drag the probe tip along thesurface after the tip is lowered to touch the surface, in an embodimentillustrated in FIG. 8B. After the probe tip has been dragged along thesurface 200 for a predetermined distance, the probe tip is again liftedby a predetermined distance, such as h, moved laterally by apredetermined distance, and then again lowered to touch the surface 200.After the tip touches the surface, the tip is again dragged along thesurface for a predetermined distance and the above-described processrepeated until a scan across the entire target area is completed asbefore. In the operational mode of FIG. 8B, in addition to recording thequantities h, δx and the distances by which the tip is repeatedlylowered before it touches the surface in the intermittent contact modethroughout the scan, system 20 also records the change in height of theprobe tip when the tip is dragged along the surface 200. Suchinformation, in conjunction with h, δx, and the distances by which thetip is lowered before it touches the surface, will give an indication ofthe geometry or profile of the surface when system 20 is operated in themode indicated in FIG. 8B.

[0067] Yet another operational mode of system 20 in the intermittentcontact mode is illustrated in FIG. 8C. Such mode is similar to that inFIG. 8A, where in the operational modes of both FIGS. 8A and 8C, theprobe tip is not moved laterally to drag the tip across the surfaceafter the tip is lowered to touch the surface, but is lifted to apredetermined height h. However, instead of moving the probe tip up anddown and laterally along substantially straight lines as in FIG. 8A, thetip in FIG. 8C is moved along a more or less sinusoidal path acrosssurface 200 until it scans across the target area. Such and othervariations are within the scope of the invention.

[0068] Where intermittent contact mode is employed, the values of δx andheight h employed in reference to FIGS. 8A-8C are chosen so that it isunlikely for the probe tip to “jump over” bumps or valleys on a surfaceto be sampled. A suitable range for h may be 10-1,000 Angstroms, and asuitable value for δx may be a fraction of the expected size of thefeature or object.

[0069] In yet another mode of operation of the apparatuses in FIGS. 2A,2B, the sensing tip may be scanned across a sample surface with the tipin contact with the surface until the feature is found. Then, the sameor a different sensing tip may be used to scan the surface with the tipin intermittent contact with the surface to measure the feature. This isillustrated in FIG. 9A. As shown in FIG. 9A, a scanning probe 300includes a stylus arm 318 and a sensing tip 320 attached to the stylusarm at or near one end of the arm. At the tip of sensing tip 320 is aflexible sensing tip 322 such as a nanotube. Thus, the spatialrelationship between sensing tip 320 and flexible tip 322 is known, sothat if a feature of interest is located by means of the sensing tip,the other sensing tip may be accurately positioned above the featurefound and used to measure the feature without again having to find thefeature. Thus, in one embodiment, arm 318 maybe scanned across a samplewith sensing tip 320 in contact with the surface to find the feature.After the feature has been found, the flexible tip 322 may be used in anintermittent contact or contact mode to measure the feature. Since theflexible tip in the form of a nanotube is long and thin, it isparticularly suitable for measuring high-aspect ratio features. Whensensing tip 320 is used to measure or find a feature in a contact mode,the flexible tip simply buckles so that sensing tip 320 may be usedduring the contact mode scan as if the flexible tip is not present.Nanotubes are very flexible and will not significantly affect theoperation of the sensing tip 320 in contact mode. After the feature hasbeen found, sensing tip 320 is withdrawn from the surface so that theflexible tip such as a nanotube will snap back to the original geometryas shown in FIG. 9A and will be in a position to be used for sensing andmeasuring the feature found.

[0070]FIG. 9B illustrates an alternative embodiment to that of FIG. 9A.Instead of mounting two sensing tips on the same common probe, twoseparate probes 318 a and 318 b may be used and two different sensingtips 330 and 332 mounted respectively onto probes 318 a, 318 b may beused where the spatial relationship between the two sensing tips isknown. Therefore, one sensing tip, such as sensing tip 330 may be usedin a contact or intermittent contact mode to find the feature. Since thespatial relationship between the two tips is known, sensing tip 332 maythen be readily positioned accurately above the feature found formeasuring the feature in a contact or intermittent contact mode. Theroles of the two tips may of course be reversed so that sensing tip 332may be used for finding the feature and sensing tip 330 may be used formeasuring the feature.

[0071] While the invention has been described above by reference tovarious embodiments, it will be understood that changes andmodifications may be made without departing from the scope of theinvention, which is to be defined only by the appended claims and theirequivalents.

What is claimed is:
 1. A method for sensing a sample employing aprofiler, said profiler having a stylus sensor assembly with an armrotatable about a pivot and a controller controlling a force acting onthe arm, said method comprising the steps of: (a) positioning a sensingtip above one location of the sample; (b) reducing a distance betweenthe sample and a sensing tip without substantially moving the tip andthe sample laterally relative to each other and without substantiallyrotating the arm about said pivot, until the tip touches the sample; (c)measuring data related to a height of the sample surface with the tipstationary and in contact with the sample; (d) increasing a distancebetween the tip and the sample to lift the tip off the sample; (e)causing lateral relative motion between the sensing tip and the sampleand positioning the tip so that the tip is above a location of thesample adjacent to and spaced apart from said one location; and (f)repeating steps (b) through (e) at a plurality of locations of thesample to obtain an image of the sample.
 2. The method of claim 1,further comprising a step (b1) of determining whether the tip touchesthe sample by comparing to a threshold value a distance rotated by thetip about the pivot caused by the sample.
 3. The method of claim 2,further comprising setting the threshold value to not more than about500 nm.
 4. The method of claim 1, wherein step (d) increases thedistance between the tip and the sample to a value greater than anexpected maximum height variation of the sample surface.
 5. The methodof claim 2, said method further comprising: sensing when a force betweenthe tip and the sample reaches a preset value; and maintaining the forcebetween the tip and the sample substantially at said preset value whenthe tip is rotated by the sample until the distance rotated issubstantially equal to said threshold value.
 6. The method of claim 5,said profiler comprising at least one moving stage for moving the tip insteps (a), (b), (d), (e), wherein said steps (a), (b), (d), (e) areperformed by the at least one stage, further comprising detecting whenthe distance that the tip is rotated by contact with the sample issubstantially equal to said threshold value.
 7. The method of claim 6,further comprising causing the moving stage to stop moving the tip whenthe distance that the tip is rotated by contact with the sample issubstantially equal to said threshold value.
 8. The method of claim 1,wherein step (e) causes lateral relative motion for a distance in therange of about 1 nm to 50 mm.
 9. The method of claim 1, said profilercomprising at least one moving stage for moving the tip in steps (a),(b), (d), (e), wherein said steps (a), (b), (d), (e) are performed bythe at least one stage.
 10. The method of claim 9, wherein said steps(a), (b) and (d) are performed without the at least one stagesubstantially moving the tip and the sample laterally relative to eachother.
 11. The method of claim 9, said data in step (c) being measuredby taking into account a distance moved by the at least one moving stagein step (b) and a distance rotated by the tip about the pivot when thetip touches the sample.
 12. The method of claim 1, wherein said arm isconstrained to substantially one degree of freedom, so that contactbetween the tip and the sample in step (b) causes the tip to rotatesubstantially without twisting.
 13. The method of claim 1, wherein saidincreasing step (c) increases the distance between the tip and thesurface to a constant value before lateral relative motion is caused instep (d).
 14. A method for sensing a feature on a surface of a sampleemploying a sensing tip, said method comprising the steps of: (a)positioning the sensing tip above one location of the surface; (b)reducing a distance between the surface and a sensing tip withoutsubstantially moving the tip and the surface laterally relative to eachother until either the tip or the surface has traveled, or the tip andthe surface together have traveled in aggregate, by a preset distancewithout contacting the surface, or until the tip touches the surface;(c) increasing a distance between the tip and the surface withoutsubstantially moving the tip and the surface laterally relative to eachother until such distance is substantially equal to a predeterminedvalue, said predetermined value being such that after step (c) the tipis higher in elevation than another location of the surface adjacent toand spaced apart from said one location; (d) causing lateral relativemotion between the sensing tip and the surface and positioning the tipso that the tip is above said another location; and (e) repeating steps(b) through (d) at a plurality of locations of the surface to find ormeasure the feature.
 15. The method of claim 14, further comprisingfinding said determined value so that after step (c) the tip is higherin elevation than another location of the surface adjacent to and spacedapart from said one location.
 16. The method of claim 15, wherein saidfinding includes measuring data related to height of the surface priorto step (c) and predicting from such data an elevation of said anotherlocation.
 17. The method of claim 16, wherein said predicting includescurve fitting said data related to heights of the surface at a pluralityof locations and extrapolation.
 18. The method of claim 16, wherein saidsample surface is inclined and wherein said predicting includesestimating a slope of the sample surface.
 19. The method of claim 15,wherein said finding step includes providing height informationconcerning a target area of the sample surface, said target areacontaining the one and the another location of the surface so that whensaid distance between the tip and said surface is increased to saidpredetermined value in step (c), the tip is at a higher elevation thanall locations within the target area of the surface.
 20. The method ofclaim 19, wherein said finding includes measuring data related to theheight of the surface at a plurality of sampling locations of thesurface within the target area.
 21. The method of claim 20, wherein saidmeasuring includes: (f) positioning the sensing tip above a firstsampling location of the surface within the target area; (g) reducing adistance between the surface and a sensing tip without substantiallymoving the tip and the surface laterally relative to each other untileither the tip or the surface has traveled, or the tip and the surfacetogether have traveled in aggregate, by a preset distance withoutcontacting the surface, or until the tip touches the surface; (h)increasing a distance between the tip and the surface withoutsubstantially moving the tip and the surface laterally relative to eachother until such distance is substantially equal to a secondpredetermined value, said second predetermined value being such thatafter step (h) the tip is higher in elevation than all locations of thesurface within the target area; (i) causing lateral relative motionbetween the sensing tip and the surface and positioning the tip so thatthe tip is above another sampling location within the target area of thesurface; and (j) repeating steps (g) through (i) at said plurality ofsampling locations of the surface within the target area.
 22. The methodof claim 14, wherein said finding includes measuring data related to theheight of the surface at 3 to 25 sampling locations of the surfacewithin the target area.
 23. The method of claim 14, wherein said causingstep (d) causes lateral relative motion over a distance in the range ofabout 1 nm to 50 mm.
 24. The method of claim 14, further comprisingmeasuring a height of the surface with the tip stationary and in contactwith the surface when the tip touches the surface.
 25. The method ofclaim 14, wherein said predetermined value is not more than about 1micron.
 26. The method of claim 14, said sensing tip being that of aprofiler, said profiler having an arm rotatable about a pivot and acontroller controlling a force acting on the arm, wherein said steps(a), (b), (c) and (d) are performed without substantially rotating thearm about said pivot unless the tip is in contact with the surface. 27.The method of claim 14, further comprising a step (b1) of determiningwhether the tip touches the surface by comparing to a threshold value adistance moved by the tip caused by the surface.
 28. The method of claim27, further comprising setting the threshold value to not more thanabout 500 nm.
 29. The method of claim 27, said method furthercomprising: sensing when a force between the tip and the sample reachesa preset value; and maintaining the force between the tip and the samplesubstantially at said preset value when the tip is moved by the sampleuntil the distance moved is substantially equal to said threshold value.30. The method of claim 29, said profiler comprising a controllercontrolling a force exerted by the tip against the surface, wherein saidcontroller is used to perform the force sensing and maintenance.
 31. Themethod of claim 14, wherein the feature is found in step (b) or step (e)when either the tip or the surface has traveled, or the tip and thesurface together have traveled in aggregate, by said preset distancewithout contacting the surface.
 32. The method of claim 31, whereinafter the feature is found, step (b) is repeated in step (e) by reducinga distance between the surface and a sensing tip until the tip touchesthe surface, said method further comprising measuring data related to aheight of the feature with the tip stationary and in contact with thesurface when the tip touches the surface to obtain a profile or an imageof the feature.
 33. The method of claim 14, wherein when the tip touchesthe surface in step (b), in the immediately following step (c), thedistance between the tip and the surface is increased by a firstpredetermined distance, and wherein when the tip does not touch thesurface in step (b), in the immediately following step (c), the distancebetween the tip and the surface is increased by a second predetermineddistance.
 34. The method of claim 33, wherein said second predeterminedvalue is substantially equal to said preset distance.
 35. The method ofclaim 33, wherein said second predetermined value is larger than thefirst predetermined value.
 36. The method of claim 14, wherein a targetarea of the sample surface has substantially two discrete heights, withportions of the surface in the target area at a lower first height andportions at a higher second height, and wherein step (a) positions thesensing tip above one location at the second height.
 37. The method ofclaim 36, wherein step (b) reduces the distance between the surface anda sensing tip until the tip touches the surface at the first or thesecond height in the target area.
 38. The method of claim 14, saidmethod employing at least one moving stage for moving the tip in steps(b), (c), (d), wherein said steps (b), (c), (d) are performed by the atleast one stage.
 39. The method of claim 38, wherein said steps (a), (b)and (c) are performed without the at least one stage substantiallymoving the tip and the sample laterally relative to each other.
 40. Themethod of claim 39, further comprising measuring data related to heightof the surface, said data being measured by taking into account adistance moved by the at least one moving stage in step (b) and adistance moved by the tip relative to the stage caused by contact withthe sample.
 41. A method for sensing a high aspect ratio feature on asurface of a sample employing a sensing tip, said method comprising thesteps of: scanning the sensing tip across the surface; and controlling adistance between the surface and the tip so that the tip contacts thesurface intermittently, without substantially moving the tip and thesurface laterally relative to each other when the tip and surface are incontact, wherein the tip does not penetrate said feature when scanningacross the surface.
 42. The method of claim 41, wherein said controllingincludes reducing a distance between the surface and the sensing tipwithout substantially moving the tip and the surface laterally relativeto each other until either the tip or the surface has traveled, or thetip and the surface together have traveled in aggregate, by a presetdistance without contacting the surface or until the tip touches thesurface.
 43. The method of claim 42, wherein the feature is found wheneither the tip or the surface has traveled, or the tip and the surfacetogether have traveled in aggregate, by said preset distance withoutcontacting the surface.
 44. The method of claim 43, further comprising,after the feature is found, measuring data related to a height of thefeature with the tip stationary and in contact with the surface when thetip touches the surface to obtain a profile or an image of the feature.45. The method of claim 42, further comprising measuring a height of thesurface with the tip held in contact with the surface when the tiptouches the surface.
 46. The method of claim 45, further comprising,after the reducing: increasing a distance between the sensing tip andthe sample without substantially moving the tip and the surfacelaterally relative to each other; and causing lateral relative motionbetween the sensing tip and the surface.
 47. The method of claim 46,wherein said sample has a sloping surface, wherein said measuringmeasures data related to height of the surface at a plurality oflocations of the surface; said method further comprising: predicting aslope of the sample surface from said data; and determining the distancethat is increased so that the sensing tip will not contact the surfacelaterally during the lateral relative motion.
 48. The method of claim41, wherein said controlling causes the distance between the surface andthe tip to increase to values not more than about 1 micron.
 49. Themethod of claim 41, said sensing tip being that of a profiler, saidprofiler having an arm rotatable about a pivot and a controllercontrolling a force acting on the arm, wherein said scanning andcontrolling steps are performed without substantially rotating the armabout said pivot unless the tip is in contact with the surface.
 50. Themethod of claim 49, further comprising determining whether the tiptouches the surface by comparing to a threshold value a distance movedby the tip about the pivot caused by the surface.
 51. The method ofclaim 50, further comprising setting the threshold value to not morethan about 500 nm.
 52. The method of claim 50, said method furthercomprising: sensing when a force between the tip and the sample reachesa preset value; and maintaining the force between the tip and the samplesubstantially at said preset value when the tip is moved by the sampleuntil the distance moved is substantially equal to said threshold value.53. The method of claim 41, said method further comprising positioningthe tip above a location of the surface so that the tip will not comeinto lateral contact with the surface.
 54. The method of claim 53,wherein said positioning includes providing height informationconcerning the surface.
 55. The method of claim 54, wherein saidproviding includes measuring height of the surface at sampling locationsof the surface in a dipping mode.
 56. A method for sensing a surface ofa sample employing a probe with a sensing tip, said method comprisingthe steps of: (a) positioning the sensing tip above one location of thesurface of the sample; (b) reducing a distance between the surface and asensing tip without substantially moving the tip and the surfacelaterally relative to each other until the tip touches the surface; (c)measuring data related to a height of the sample surface with the tip incontact with the sample at a contact point; (d) increasing a distancebetween the tip and the contact point of the surface withoutsubstantially moving the tip and the surface laterally relative to eachother until such distance is substantially equal to a predeterminedvalue; (e) causing lateral relative motion between the sensing tip andthe surface and positioning the tip so that the tip is above a locationadjacent to and spaced apart from said one location; and (f) repeatingsteps (b) through (e) at a plurality of locations of the surface toobtain an image of the surface; said method further comprising step (e1)of determining whether the tip and the surface are in contact duringstep (d), wherein steps (b) through (e) are repeated in step (f) onlyafter a determination that the tip and the surface are not in contact.57. The method of claim 56, wherein step (a) positions the sensing tipwithout adequate prior information concerning extent of heightvariations of the surface to avoid lateral contact between the tip andthe surface when the tip is scanned across the surface, and said lateralrelative motion in step (e) is over a lateral distance less than about100 nm.
 58. The method of claim 57, further comprising step (e2) ofincreasing a distance between the tip and the sample withoutsubstantially moving the tip and the sample laterally relative to eachother until such distance is substantially equal to the predeterminedvalue when said determining step (e1) determines that the tip and thesurface are in contact after step (d).
 59. The method of claim 58,further comprising repeating steps (e1), (e2) until said determiningstep determines that the tip and surface are not in contact, whereinsteps (b) through (e) are then repeated in step (f).
 60. The method ofclaim 58, further comprising discarding the data measured when the tipremains in contact with the surface after step (d) and during steps (e1)and (e2).
 61. The method of claim 56, wherein the sensing tip is that ofa probe of a profiler or scanning probe microscope, step (e1) determineswhether the tip and the surface are in contact by: causing the probe toapproach the surface; and sensing whether the tip rises as soon as theprobe is caused to approach the surface or whether the tip has moved bya threshold distance when the probe is caused to approach the surface.62. The method of claim 61, further comprising setting the thresholdvalue to not more than about 500 nm.
 63. The method of claim 61, saidprobe being that of a profiler, said method further comprising: sensingwhen a force between the tip and the sample reaches a preset value; andmaintaining the force between the tip and the sample substantially atsaid preset value when the tip is rotated by the sample about a pivot ofthe profiler until the distance rotated is substantially equal to saidthreshold value.
 64. The method of claim 56, wherein said measuringmeasures data related to a height of the sample surface with the tipstationary and in contact with the sample at the contact point.
 65. Amethod for sensing a high aspect ratio feature on a surface of a sampleemploying at least a first and a second sensing tips on a common probe,said method comprising the steps of: scanning the first sensing tipacross the surface with the first tip in contact with the surface untilthe feature is found; and scanning the second sensing tip across thesurface with the second tip in intermittent contact with the surface tomeasure the feature.
 66. The method of claim 65, wherein the secondsensing tip comprises a nanotube.
 67. A scanning probe including a firstand a second sensing stylus with known spatial relationship to eachother, said stylus including a nanotube.
 68. The probe of claim 67, saidnanotube extending beyond the first sensing stylus.
 69. A method forsensing a high aspect ratio feature on a surface of a sample employingat least one sensing tip on a probe, said method comprising the stepsof: scanning the at least one sensing tip across the surface with thetip in contact with the surface until the feature is found; and scanningthe at least one sensing tip across the surface with the tip inintermittent contact with the surface to measure the feature.
 70. Amethod for sensing a high aspect ratio feature on a surface of a sampleemploying a sensing tip of a profiler or scanning probe microscope, saidmethod comprising the steps of: providing height information of thesurface within a target area of the surface; positioning the tip aboveone location of the surface in the target area using such information;scanning the sensing tip across the surface; and controlling a distancebetween the surface and the tip using said information during thescanning so that the tip contacts the surface intermittently and so thatthe tip does not contact the surface laterally when the tip is scannedacross the surface.
 71. The method of claim 70, wherein said controllingcontrols said distance so that the tip is scanned without substantiallymoving the tip and the surface laterally relative to each other when thetip and surface are in contact, wherein the tip does not penetrate saidfeature when scanning across the surface,
 72. The method of claim 70,wherein said tip contacts the surface at a sequence of locations,wherein said providing includes measuring data related to the height ofthe surface at at least one location within a target area of the surfacewhen the tip is in contact with the surface, and wherein saidcontrolling controls the distance using said data so that the tip israised to an elevation higher than the subsequent location of thesurface in the target area in the sequence.
 73. The method of claim 72,wherein said providing includes measuring data related to the height ofthe surface at a plurality of sampling locations within the target areaof the surface.
 74. The method of claim 73, wherein said controllingincludes predicting from such data an elevation of the subsequentlocation of the surface in the target area in the sequence.
 75. Themethod of claim 74, wherein said predicting includes curve fitting suchdata and extrapolation.
 76. The method of claim 73, wherein saidmeasuring includes: (a) positioning the sensing tip above a firstsampling location of the surface in the target area of the surface; (b)reducing a distance between the surface and a sensing tip withoutsubstantially moving the tip and the surface laterally relative to eachother until either the tip or the surface has traveled, or the tip andthe surface together have traveled in aggregate, by a preset distancewithout contacting the surface, or until the tip touches the surface;(c) increasing a distance between the tip and the surface withoutsubstantially moving the tip and the surface laterally relative to eachother until such distance is substantially equal to a secondpredetermined value, said second predetermined value being such thatafter step (c) the tip is higher in elevation than all locations of thesurface within the target area; (d) causing lateral relative motionbetween the sensing tip and the surface and positioning the tip so thatthe tip is above another sampling location in the target area; and (e)repeating steps (b) through (d) at said plurality of sampling locationsof the surface in the target area.
 77. An apparatus for sensing asample, comprising: a senor assembly for sensing the sample, saidassembly including a base portion and a movable portion, said movableportion including a sensing tip connected to the base portion, wherein aforce applied to the tip caused by contact between the tip and thesample may cause the tip to move relative to the base portion; one ormore moving stage(s) causing a vertical relative motion between theassembly and the sample, thereby changing a distance between theassembly and the sample; and a measurement controller computing a changein the distance between the sensing tip and the sample caused by acombination of the relative motion between the tip and the base portionof the sensor assembly and said vertical motion.
 78. The apparatus ofclaim 77, said assembly comprising an arm rotatable about a pivot, saidarm having a sensing tip, and a sensor sensing distance rotated by saidsensing tip about the pivot; wherein the measurement controller computesa change in the distance between the sensing tip and the sample causedby a combination of the rotation of the tip about the pivot and saidvertical motion.
 79. The apparatus of claim 78, further comprising afeedback path between the sensor and the moving stage(s), said pathtransmitting a signal representative of the distance moved by the tipcaused by the surface, said one or more moving stage(s) controlling thevertical motion in response to said signal.
 80. The apparatus of claim79, said moving stage(s) having a motion controller controlling thevertical motion in response to the signal.
 81. The apparatus of claim80, wherein said motion controller causes the moving stage(s) to stopcausing vertical motion between the profiler and the sample when the tiphas moved by a distance substantially equal to a threshold value. 82.The apparatus of claim 78, further comprising a force controllercontrolling a force acting on the sensor assembly so that a preset forceis applied by the tip to the sample when the tip is caused to move bythe surface.